Impact of fabric, microcracks and stress field on shale anisotropy

Abstract

Few data are available on shales in terms of seismic to ultrasonic properties and anisotropy, although all are important with regards to imaging problems often encountered in such lithologies. Additionally, mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions. The results presented here are derived from a set of experiments run on Muderong Shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response. This shale was noted to have a linear velocity-mean effective stress response and extremely high anisotropy, both likely the result of the presence of fluid-filled cracks in a low- permeability porous medium. Changes in velocity and Vp/Vs ratios are dependent on both stress and smectite content. S-wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress (<20 MPa) is less sensitive to stress change than P-wave velocity. Vp/Vs ratios are noted to increase in this shale up to 20 MPa effective stress, then decrease slightly due to stress-induced loss of interlayer water in smectite. Intrinsic anisotropy comes from composition, a strong compaction fabric and the presence of microfractures; changes to ultrasonic anisotropy are the result of the magnitude of the stresses, their orientation with respect to the fractures and the degree of stress anisotropy.